The present invention relates to a belt pressure filter suited for treatment of a sludge or the like produced in various types of water treatment facilities. More specifically, the present invention relates to a belt pressure filter which is capable of an automatic treatment in such a manner that in spite of variations in the sludge characteristics, such as the concentration of organic substances, the water content of a dewatered cake may be maintained relatively low and constant while the dosage of a coagulating agent that is added to a raw sludge solution may be maintained at an optimum and an abnormal condition, such as poor coagulation can be readily discovered.
A belt pressure filter is a kind of a dewatering machine often used in a dewatering process and may be classified as a filtration type dewatering machine using a filter belt. Since a belt pressure filter employs a belt press type sludge dewatering system using mesh like filter belts and rolls, the driving power may be small and any increase in the amount of solid matter through injection of an agent, is small, so that a cake having a small water content can be obtained. Therefore, attention has been attracted to such a system since it fully meets the requirement of saving energy. Generally, a belt pressure filter comprises a gravity dewatering zone for dewatering a sludge by gravity, a roller press dewatering zone for dewatering the sludge by means of a roller press, a compressive dewatering zone for dewatering the sludge by compressive force, and a shearing stress dewatering zone for dewatering the sludge by a shearing stress. A belt pressure filter comprises basically two mesh-like filter belts and rollers. This type of belt pressure filter needs to take into consideration, as the factors being controlled, four factors, i.e. the filter belt traveling speed, the sludge concentration as a typical factor representing a sludge characteristic dosage of an agent being dosed or a dosage of a coagulating agent, and the amount or quantity of sludge being supplied. It is desired that these factors are properly controlled so that the water content of a dewatered cake is maintained low and constant and in addition the dosage of a coagulating agent is maintained as low as possible. The present invention achieves such purposes.
More specifically, the characteristics of a material being supplied to a dewatering machine for the purpose of processing may vary. The amount or concentration of solids in a sludge or the amount of organic substance and the number of particles in a sludge may vary. Therefore, even if the amount of supplied material to be processed is maintained constant, a difference in the filtration amount may be caused in the gravity dewatering zone. Accordingly, the thickness of the material supplied from a source material supply tank onto a filter belt may be changeable. Usually, it is desired to make uniform the layer thickness throughout the width of the material as it is supplied from the gravity dewatering zone onto a forced dewatering zone comprising a roller press dewatering zone. Therefore, rollers are provided at the entrance of the forced dewatering zone for the purpose of adjusting the layer thickness of the material being processed. However, other means for adjusting such layer thickness may be provided. Accordingly, if and when the concentration of the solids in the sludge material becomes too high and/or a poor coagulation is present or when the amount of sludge being processed increases, the material being processed may cause a congestion at the roller for adjusting the layer thickness, which could cause a leakage of the material. On the other hand, if and when the amount of the material being processed is decreased, i.e. when the concentration of the solid material is low and/or a poor coagulation is present, it could happen that a desired water content cannot be attained. Thus, there is a problem that the dewatering performance or efficiency is lowered due to a change in the sludge characteristics mainly due to a change in the concentration of the solid material components of the sludge. If the concentration of solids changes, it is necessary to determine the dosage of a coagulating agent in association with or as a function of the solids concentration. If the dosage of a coagulating agent is maintained constant, it could happen that the dewatering performance or efficiency is lowered or too large a dosage of a coagulating agent exceeding a required amount would cause an uneconomical waste of agent.